Conformational dynamics of amyloid β-protein assembly probed using intrinsic fluorescence

Formation of toxic oligomeric and fibrillar structures by the amyloid beta-protein (A beta) is linked to Alzheimer's disease (AD). To facilitate the targeting and design of assembly inhibitors, intrinsic fluorescence was used to probe assembly-dependent changes in A beta conformation. To do so, Tyr was substituted in A beta 40 or A beta 42 at position 1, 10 (wild type), 20, 30, 40, or 42. Fluorescence then was monitored periodically during peptide monomer folding and assembly. Electron microscopy revealed that all peptides assembled readily into amyloid fibrils. Conformational differences between A beta 40 and A beta 42 were observed in the central hydrophobic cluster (CHC) region, Leu(17)-Ala(21). Tyr(20) was partially quenched in unassembled A beta 40 but displayed a significant and rapid increase in intensity coincident with the maturation of an oligomeric, a-helix-containing intermediate into amyloid fibrils. This process was not observed during A beta 42 assembly, during which small decreases in fluorescence intensity were observed in the CHC. These data suggest that the structure of the CHC in beta P42 is relatively constant within unassembled peptide and during the self-association process. Solvent accessibility of the Tyr ring was studied using a mixed solvent (dimethyl sulfoxide/water) system. [Tyr(40)]A beta 40, [Tyr(30)]A beta 42, and [Tyr(42)]A beta 42 all were relatively shielded from solvent. Analysis of the assembly dependence of the sitespecific intrinsic fluorescence data suggests that the CHC is particularly important in controlling A beta 40 assembly, whereas the C-terminus plays the more significant role in A beta 42 assembly.